The purpose of this protocol is to determine the mechanism of weight gain during adjuvant chemotherapy in patients with metastatic breast cancer. Twenty-four premenopausal, non-obese women with invasive localized breast cancer receiving adjuvant chemotherapy will be studied pre- and following six cycles of therapy. Studies include determination of resting metabolic rate following on overnight fast in 60-minute recumbant test using indirect calorimetry. Serum samples will be drawn for a variety of blood, tests including estrogen and thyroid levels.